1. Field of the Invention
The present invention relates to a negative pressure type booster for use in, for example, a vehicular brake system.
2. Description of the Related Art
A negative pressure type booster of the above-mentioned type is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 08-310379. The disclosed negative pressure type booster includes a housing; a movable wall mounted in the housing in a frontward/rearward movable condition and dividing the interior of the housing into a constant-pressure chamber and a variable-pressure chamber; a power piston connected to the movable wall in a frontward/rearward movable condition; an input member provided in the power piston in a frontward/rearward movable condition relative to the power piston and receiving an external operation force applied thereto in a frontward direction; an output member for outputting, to an external device, a thrust force generated frontward by the power piston; a control valve mounted in the power piston and comprising an atmospheric control valve portion and a vacuum control valve portion, the atmospheric control valve portion facing frontward, toward an atmospheric valve seat provided on the input member, and adapted to establish/shut off communication between the variable-pressure chamber and the atmosphere in cooperation with the atmospheric valve seat, and the vacuum control valve portion facing frontward, toward a vacuum valve seat provided on the power piston, and adapted to establish/shut off communication between the variable-pressure chamber and the constant-pressure chamber in cooperation with the vacuum valve seat; first biasing means for biasing the control valve frontward toward the atmospheric valve seat and the vacuum valve seat; second biasing means for biasing the input member rearward; and a stopper for limiting the quantity of movement of the input member relative to the power piston effected by biasing force of the second biasing means. Notably, when the negative pressure type booster is applied to, for example, a vehicular brake system, the term “rearward” refers to the side toward a brake pedal or the side toward the vehicular rear, and the term “frontward” refers to the side toward a brake master cylinder or the side toward the vehicular front.
In the negative pressure type booster disclosed in above-mentioned Japanese Patent Application Laid-Open (kokal) No. 08-310379, the input member includes a plunger that is mounted to the power piston in a frontward/rearward movable condition, and an input rod connected to the plunger. The atmospheric valve seat is formed at a rear end portion of the plunger. In such a configuration, when the atmospheric valve seat and the atmospheric control valve portion are in contact with each other to thereby shut off communication between the variable-pressure chamber and the atmosphere, a differential pressure between a vacuum in the variable-pressure chamber and the atmospheric pressure is exerted on the area of an atmospheric valve as measured with the effective diameter of the atmospheric valve (pressure reception area). A differential-pressure-induced force pushes the input member frontward. In an ordinary state (in a non-operational state; hereinafter the same applies), in order to maintain contact between the atmospheric valve seat and the atmospheric control valve portion, the biasing force of the second biasing means must be set greater than the differential-pressure-induced force.
In a negative pressure type booster of the above-mentioned type, in an ordinary state where the atmospheric valve seat is in contact with the atmospheric control valve portion, and the vacuum control valve portion is separated from the vacuum valve seat, a vacuum in the variable-pressure chamber may vary due to, for example, a variation in pressure of a vacuum source. Thus, the biasing force of the second biasing member is determined so as to cope with a potential high vacuum associated with the vacuum variation in the variable-pressure chamber. As a result, in an ordinary state, if a vacuum in the variable-pressure chamber becomes low, an operation force required to separate the atmospheric valve seat from the atmospheric control valve portion; i.e., startup load, will increase, giving an operator (a vehicle driver) a sensation of unusual operation. This problem arises eminently in the case where the effective diameter of the atmospheric valve is increased in order to obtain high operational response, as in the case of the above-mentioned negative pressure type booster disclosed in the above-described patent publication.